In batteries, for example, lithium ion batteries, thermal runaway is a potential problem. Thermal runaway may be initiated by, among other things, physical contact between the anode and cathode of the battery, due to the internal force created by the volume changes of the anode and the cathode during normal cycling, which, in turn, causes a rapid evolution of heat. The rapid evolution of heat may cause ignition of thermal chemical reactions of anode/electrolyte, cathode/electrolyte, anode/cathode, or electrolyte/electrolyte. The ignition scenario may lead to a hazardous situation for the battery.
Thermal runaway may be categorized as ‘sudden’ thermal runaway or ‘delayed’ thermal runaway. Sudden thermal runaway refers to very rapid heat evolution, i.e., arising in less than 1 second after inception. Delayed thermal runaway refers to heat evolution, i.e., arising in more than 3 seconds after inception. In lithium ion batteries, over 99% of failures are caused by sudden thermal runaway. Delayed thermal runaway may be safeguarded against by the use of a ‘shutdown’ separator (e.g., a separator that responds to increasing heat by pore closure that stops ionic flow between the anode and cathode), or by the rapid dissipation of heat from the cell. Sudden thermal runaway, however, has not been successfully dealt with.
Sudden thermal runaway of the Li-ion cells may be simulated in battery safety tests referred to as: the ‘nail penetration’ test or the ‘crush’ test (crush tests include: ball crush, bar crush, and plate crush). In each of these tests, an external force, applied via a nail, ball, bar, or plate, is exerted on the housing (or ‘can’) of the battery which, in turn, may cause the anode and cathode to come into physical contact.
The foregoing safety tests exacerbate the tight fitting situation already existing within the battery housing. For example, lithium ion batteries are, most often, produced in cylindrical and prismatic forms. The anode/separator/cathode are wound or folded, without electrolyte, into shape and then snuggly fit into their housing (can) and capped shut. When electrolyte is added, the anode/separator/cathode swell. This causes internal forces within the can to increase. Later, during ‘formation’ (i.e., when the battery is given an initial charge), the anode and cathode expand again (e.g., the anode may expand by about 10% and the cathode may expand by about 3%). The expansion during formation again causes internal forces within the can to increase. These internal forces, such as those from the nail penetration and crush tests mentioned above, are directed toward the center of the battery. When the external forces are exerted on the can, those forces are also directed toward the center of the battery. The result is extraordinary pressures within the battery and those pressures are forcing the anode and cathode into physical contact by compressing the microporous membrane separator placed therebetween.
The use of microporous membranes as battery separators is known. For example, microporous membranes are used as battery separators in lithium ion batteries. Such separators may be single layered or multi-layered thin films made of polyolefins. These separators often have a ‘shut-down’ property such that when the temperature of the battery reaches a predetermined temperature, the pores of the membrane close and thereby prevent the flow of ions between the electrodes of the battery. Increasing temperature in the battery may be caused by internal shorting, i.e., physical contact of the anode and cathode. The physical contact may be caused by, for example, physical damage to the battery, damage to the separator during battery manufacture, dendrite growth, excessive charging, and the like. As such, the separator, a thin (e.g., typically about 8-25 microns thickness) microporous membrane, must have good dimensional stability.
Dimensional stability, as it applies to battery separators, refers to the ability of the separator not to shrink or not to excessively shrink as a result of exposure to elevated temperatures. This shrinkage is observed in the X and Y axes of the planar film. This term has not, to date, referred to the Z-direction dimensional stability.
Puncture strength, as it applies to battery separators, is the film's ability to resist puncture in the Z-direction. Puncture strength is measured by observing the force necessary to pierce a membrane with a moving needle of known physical dimensions.
To date, nothing has been done to improve the Z-direction dimensional stability of these battery separators. Z-direction refers to the thickness of the separator. A battery is tightly wound to maximize its energy density. Tightly winding means, for a cylindrically wound battery, that forces are directed radially inward, causing a compressive force on the separator across its thickness dimension. In the increasing temperature situation, as the material of the separator starts to flow and blind the pores, the electrodes of the battery may move toward one another. As they move closer to one another, the risk of physical contact increases. The contact of the electrodes must be avoided.
Accordingly, there is a need for a battery separator, particularly a battery separator for a lithium ion battery, having improved Z-direction stability, and for a battery separator that can prevent or reduce failure arising from sudden thermal runaway.
In the prior art, it is known to mix filler into a separator for a lithium battery. In U.S. Pat. No. 4,650,730, a multi-layered battery separator is disclosed. The first layer, the ‘shut down’ layer, is an unfilled microporous membrane. The second layer, the dimensionally stable layer, is a particulate filled microporous layer. The second layer, in final form (i.e., after extraction of the plasticizer), has a composition weight ratio of 7-35/50-93/0-15 for polymer/filler/plasticizer. There is no mention of Z-direction dimensional stability; instead, dimensional stability refers to the length and breadth dimensions of the separator. The filler is used as a processing aid so that the high molecular weight polymer can be efficiently extruded into a film. In U.S. Pat. No. 6,432,586, a multi-layered battery separator for a high-energy lithium battery is disclosed. The separator has a first microporous membrane and a second nonporous ceramic composite layer. The ceramic composite layer consists of a matrix material and inorganic particles. The matrix material may be selected from the group of polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE) polyurethane, polyarcylonitrile (PAN), polymethylmethacrylate (PMMA), polytetraethylene glycol diacrylate, copolymers thereof and mixtures thereof. The inorganic particles may be selected from the group of silicon dioxide (SiO2), aluminum oxide (Al2O3), calcium carbonate (CaCO3), titanium dioxide (TiO2), SiS2, SiPO4, and the like. The particulate makes up about 5-80% by weight of the ceramic composite layer, but most preferably 40-60%. There is no mention of Z-direction stability, and the particulate is chosen for its conductive properties.